1. Field of the Invention
The present invention relates to the technical field of preparing an inorganic porous catalytic material. Specifically, the present invention relates to a method for preparing titania or precursor thereof with microporous-mesoporous structure, which has the advantages of controllable pore structure parameters, such as the most probable pore size, specific surface area and pore volume, simple and controllable synthesis conditions, and short synthesis time.
2. Related Art
The pore structure is one of the important indexes of catalyst. It is generally believed that it is necessary to accelerate the diffusion rate or try to shorten the diffusion path to improve the utilization rate of the active site of the hole. However, the pore sizes of the porous catalysts are different for different reactants and reaction conditions in practice. On one hand, increasing the pore size of the catalyst not only can enlarge the region into which the functional groups such as metal oxide and metal cluster with large molecule weight and volume enter, but also provide a larger place for the ion and atom cluster into the pore path, thus facilitating the diffusion of the reactant and solvent molecule in the pore path, thereby improving the reaction conversion and catalyst performance. On the other hand, the catalyst with small pore size has the characteristic of shape-selective reaction, thus inhibiting other side reactions in the reaction system, thereby improving the reaction specificity and selectivity greatly. The pore volume and specific surface area are also the important physical properties of the porous catalyst. However, how the pore volume and specific surface area impact the catalyst activity is unclear till now: generally, increasing the pore volume and specific surface area of the catalyst is beneficial for the activity; but some catalysts with small pore volumes and specific surface areas have high activities. Therefore, it seems particularly important to prepare the porous material suitable for the actual reaction system.
Porous titania, especially microporous and mesoporous titania attract particular attention due to high specific surface area and high activity in hydrogen storage, fuel cell, photocatalytic degradation of organic materials, solid acid catalysis etc., due to its. There are also various studies on the application of microporous-mesoporous titanic acid in lithium ion battery, environmental monitoring, photocatalysis, and organic catalysis. However, how to obtain microporous-mesoporous titania or precursor thereof with controllable pore structure has always been a problem. 1) When adjusting the pore structure within a narrow range by varying conditions of sol-gel reaction or post-treatment, such as pH value, water amount, application of magnetic field, microwave and roasting temperature, the conditions must be controlled very precisely. 2) If adding templating agents with different sizes or surfactant, phosphoric acid, nitric acid, aqueous ammonia etc. as catalysts or structure directing agents in the preparation of titania sol, this process is merely suitable for a small scale preparation, furthermore, treatment of these catalysts may cause environmental pollution. 3) As titania or precursor thereof with high specific surface area cannot be obtained from titanate with routine or conventional methods, according to an intercalation or reassembly method employed by Sasaki et al., titanate or titania is used as raw material, alumina or organic ammonium salt is used as supporting material, and the pore structure is controlled by varying amounts of these materials, but the raw material of this method is expensive, and the scheme is complex. 4) According to the finding of Wellenberg et al., the specific surface area of the product titania or precursor thereof can be increased effectively, when a small amount of potassium dititanate is present in the titanate raw material, but no further study has been conducted.
To solve the above problems, the inventors have developed a new scheme (ZL 03158274.5) for preparing mesoporous titania with high specific surface area with potassium titanate as raw material. This method is different from the conventional sol-gel method, and has the advantages that the raw materials are cheap, no templating agent or surfactant is added, the production cost is low, and the process is simple and can be easily enlarged. According to this method, as raw material, a titanium compound and a potassium compound are mixed uniformly at the molar ratio of TiO2/K2O=1-4, in which the titanium compound and the potassium compound are converted into TiO2 and K2O. The mixture is sintered at 600-1100° C. for more than 30 min to obtain the product potassium titanate containing the potassium dititanate crystal. After being dispersed in water, the sintered product is added into alkaline solution (the volume ratio of the solution and product was 1-50) with a pH value of equal to or higher than 8 or a K+ solution having a concentration of higher than 1 mol/L and reacted for more than 4 h at 20-100° C. Then, the reaction solution is subjected to hydration treatment in an acid solution with a pH value of lower than 5, to give a hydrated product with a potassium content of lower than 5 wt % based on the product weight and a specific surface area of higher than 100 m2/g). After being dehydrated by thermal treatment, could the product crystallize into hydrated titania, octatitanic acid, Monoclinic-type titania, anatase-type titania, rutile-type titania, or a mixture of more than one titania above. The specific surface area of this product is higher than 50 m2/g, and the crystal form is crystal whisker with a diameter or equivalent diameter of 0.1-10 μm.
However, this method still has some disadvantages. 1) The pore size of this product merely remains at about 10 nm and cannot be controlled as needed, and nor can be diminished into the microporous range, furthermore, the adjusting of the pore size within a narrow range is realized on the basis of the expense of the degree of crystallinity. 2) As the aqueous solution is necessarily used, an excessively long production cycle is caused, e.g. 7 to 10 days for the hydration process, and the amount and composition (pH, ion concentration, etc.) are required to be controlled stringently. 3) As the specific surface area of the product is regulated mainly by varying the sintering condition in the earlier stage, the process gets complicated, and is influenced significantly by uncertain factors, such as temperature field distribution, thus the product quality is unstable. 4) The source of the raw material is limited, because the raw material is required to contain potassium dititanate. If the above problems can be solved, the industrial production of titania with high specific surface area can be realized actually.